L2CAP: Decouple scheduler and segmenter

Scheduler is used for enqueuing PDUs from segmenters to link queue end.
Segmenter is used for dequeuing SDUs from channels to scheduler.
Segmenter needs to handle converting PDUs into SDUs.

Bug: 144375926
Test: run_cert.sh and bluetooth_test_gd
Change-Id: Ia4a1707d7e0147f9f63c137e62d15ecdc877a813